1. Planaria - Protonephridia
2. Branchiostoma - Protonephridia
3. Earthworm - Nephridia.
4. Cockroach - Malpighian tubule
5. Crayfish - Green gland
6. Pisces, amphibians, reptiles, birds and mammals - Kidneys

The human excretory system comprises of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
1. Kidneys - There is a pair of kidneys which are dark-red, each with bean-shaped with a notch, the hilus is present on its inner side. The blood vessels, ureter enter or leave the kidney through hilus. Each kidney  is about 5 cm wide and 3 cm thick. In adult male the weight of the kidney is 150 gms and in the adult female, it weighs about 135 gms. They are situated in the anterior part of the abdomen protected by the pair of floating ribs (last two ribs). The left kidney is slightly lower than the left kidney.
2. Ureters - The ureters of each kidney leaves from the renal pelvis in hilus region. They are narrow tubular structures that connect the kidney to the urinary bladder. The length of each ureter is about 20 to 30 cm.
3. Urinary bladder - It is a pear-shaped muscular structure having inner linings made of transitional epithelium. The well developed muscular layer of the urinary bladder is called detrusor muscle.The urinary bladder has three openings - two openings to which ureters are connected and one opening connected to the urethra.
4. Urethra - The urethra is a canal-like structure that extends from the neck of the bladder. In females, the length of the urethra is short (about 4cm) and carries only urine. In males, the urethra is much longer (about 20cm) and carries both urine and semen.

1. There is a pair of kidneys which are dark-red, each with bean-shaped with a notch, the hilus is present on its inner side.
2. The blood vessels, ureters enter or leave the kidney through the hilus.
3. Each kidney is about 5 cm wide and 3 cm thick.
3. In an adult male, the weight of the kidney is 150 gms and in the adult female, it weighs about 135 gms.
4. They are situated in the anterior part of the abdomen protected by the pair of floating ribs (last two ribs).
5.The left kidney is slightly lower than the left kidney.
6. Each kidney is made up of three layers renal cortex, renal medulla and renal pelvis.
7. The renal medulla has 15 to 16 conical structures called medullary pyramids. The renal medullary pyramid ends in a structure called the renal papilla. 
8. Columns of Bertini are present between the medullary pyramids.
9. The medullary pyramids are connected to minor calyces. the minor calyces lead to major calyces.
10. The major calyces open into the renal pelvis. The renal pelvis leads to the ureter.
11. A kidney has about 10 lakh structural and functional units called nephrons.

A nephron is a microscopic structural and functional unit of the kidney. It is made of a renal corpuscle and a renal tubule. The renal corpuscle consists of a network of capillaries called glomerulus and Bowman's capsule. The corpuscle and tubule both are connected. They are made of epithelial cells. The tubule has five parts, namely:
1. Proximal convoluted tubule which is connected to the Bowman's capsule.
2. The loop of Henle which has two parts, ascending loop of Henle and descending loop of Henle.
3. Distal convoluted tubule
4. The collecting tubule
5. Collecting ducts

The urine formation in the nephron takes involves three steps:
1. Glomerular filtration - The semi-permeable glomerular capillaries act as filters. It allows water, glucose, salts, amino acids and other nitrogenous waste materials to pass through it and enter the Bowman's capsule. The filtrate so formed is called glomerular filtrate. 
2. Tubular reabsorption - The filtrate in the Bowman's capsule enters the Proximal convoluted tubule (PCT). About 65 % of glomerular filtrate is reabsorbed in PCT. Glucose, amino acid, vitamins, hormones, various salts, water, and some urea from the filtrate is absorbed. The filtrate reaches Loop of Henle which consist of descending and ascending limb. As the filtrate flows through the descending loop of Henle only water is reabsorbed. Sodium and other solutes are not absorbed here. Further, the filtrate flows through the ascending loop of Henle where reabsorption of sodium, potassium, calcium, magnesium, and chloride are reabsorbed. The ascending loop of Henle is impermeable to water, hence no water is reabsorbed in this region. The filtrate then flows through the Distal convoluted tubule(DCT). In DCT, active reabsorption of sodium takes place under the influence of aldosterone. Chloride ions are also reabsorbed. Water is reabsorbed under the influence of ADH(antidiuretic hormone). Passing through the DCT, the filtrate enters the collecting ducts. Further reabsorption of water takes place in the collecting ducts.
3. Tubular secretion - Secretion is the final step in the formation of urine. Creatinine, hippuric acid, drugs etc are actively secreted into the filtrate In the proximal convoluted tubule. Urea enters the filtrate by diffusion in the thin segment of ascending loop of Henle. Potassium, hydrogen, and bicarbonate ions are secreted into the filtrate in the DCT. Removal of hydrogen ions and ammonia from the blood in PCT and DCT helps to maintain the pH of blood. 
4. Thus, the urine created by this process then passes to the central part of the kidney called the pelvis. The urine then passes from pelvis to urinary bladder through ureters. 

Micturition is regulated by the following:
Control by antidiuretic hormone (ADH) 
1. ADH produced in the hypothalamus of the brain and released into the bloodstream from the pituitary gland, enhances fluid retention by making the kidneys reabsorb more water.
2. The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood above a set point of 300 mos mL${}^1$.
Control by juxtaglomerular apparatus (JGA) 
1. JGA operates a multihormonal Renin-Angiotensin-Aldosterone system (RAAS).
2. The JGA responds to a decrease in blood pressure or blood volume in the afferent arteriole of the glomerulus and releases an enzyme, renin, into the bloodstream. 
3. In the blood, renin initiates chemical reactions that convert a plasma protein, called angiotensinogen, to a peptide, called angiotensin II, which works as a hormone. 
4. Angiotensin II increases blood pressure by causing arterioles to constrict. It also increases blood volume in two ways: firstly, by signaling the proximal convoluted tubules to reabsorb more NaCl and water, and secondly, by aldosterone, a hormone that induces the distal convoluted tubule to reabsorb more $N{a}^{+}$ and water. 
Control by anti-natriuretic factor (ANF) 
1. A peptide called Atrial Natriuretic Factor (ANF), opposes the regulation by RAAS. 
2. The walls of the atria of the heart release ANF in response to an increase in blood volume and pressure. 
3. ANF inhibits the release of renin from the JGA, and thereby inhibits NaCl reabsorption by the collecting duct and reduces aldosterone release from the adrenal gland. 
4. Thus ADH, RAAS, and ANF provide an elaborate system of checks and balances that regulate kidney functioning, to control body osmolarity, salt concentrations, blood pressure, and blood volume.

1. Uremia
2. Renal failure
3. Hypertension
4. Renal calculi
5. Nephritis
6. Renal tubular acidosis
7. Diabetis insipidus, etc